1. Field of the Invention
The present invention relates to an optical information reproducing apparatus, and more-particularly to an optical information reproducing apparatus which reproduces information by focusing an information recording plane of an information record medium onto a photo-detector by a focusing optical system having an optical axis inclined to a normal line to the information recording plane.
2. Related Background Art
Recently, there has been great demand to increase the density of information recording. To satisfy such a request, an optical information recording and reproducing system has been utilized. In the optical information recording and reproducing system, since an information unit for recordal may be an illumination spot formed by light irradiation through an optical system, the recording density is very high. Further, since record/reproduce means need not be contacted to the record medium to record and reproduce information, abrasion of the record medium and recording and reproducing means is eliminated.
The record medium used in such an optical information recording and reproducing system may be of disk-shape, card-shape or tape-shape, but a card-shaped record medium (optical card) is preferable from a standpoint of portability, and the range of application is expected to more and more expand.
Such optical cards may be one on which information is recorded by the direction of magnetization and in which information can be repeatedly written, read or erased, one on which information is recorded by a pit pattern and in which information may be additionally written and read, and one which is a read-only type in which information is recorded by reflection factor. Those cards are used depending on their characteristics.
When a large number of cards having the same information recorded thereon are to be manufactured, the read-only optical cards (optical ROM cards) which have a high S/N ratio and are easy to manufacture are preferably used.
The optical ROM card is manufactured in the following manner. On a plastic substrate having a thickness of several hundreds .mu.m, an evaporated film of a low reflection factor metal such as Te is formed, a mask corresponding to a record information pattern is mounted on the evaporated film, a high reflection factor metal such as Cu is formed thereon, and an optically transparent plastic film having a thickness of several hundreds .mu.m is laminated on the low reflection factor metal film and the patterned high reflection factor metal film to form a protective film.
FIG. 1 shows an optical head of a conventional reproducing apparatus used to reproduce information recorded on such an optical ROM card.
In FIG. 1, numeral 2 denotes an optical ROM card which comprises a substrate 2a, an information record plane 2b and a transparent protective layer 2c. Numeral 4 denotes an illumination light source which may be an LED or a semiconductor laser. Numeral 6 denotes a condenser lens of an illumination optical system, and numeral 6a denotes an optical axis thereof. The optical axis inclines by an angle .theta. with respect to the information record plane 2b of the optical ROM card 2. Numeral 8 denotes a photo-detector which may be a CCD line sensor. Numeral 10 denotes a focusing lens of a focusing optical system and numeral 10a denotes an optical axis thereof. The optical axis inclines by the angle .theta. with respect to the information record plane 2b of the optical ROM card 2. The optical axis 10a and the optical axis 6a of the condenser lens 6 are symmetric with respect to a normal line 12 to the information record plane of the optical ROM card 2.
The illumination light emitted from the light source 4 is focused by the illumination optical system and it passes through the protective layer 2c of the optical ROM card 2 and illuminates a predetermined area on the information record plane 2b. The predetermined area of the information record plane 2b thus illuminated is focused onto the photo-detector 8 by the focusing optical system.
On the information record plane 2b of the optical ROM card 2, an appropriate number of record bits which are information units are arranged in a direction B to form an information track, and an appropriate number of such information tracks are arranged in a direction A to form a band, and an appropriate number of such bands are arranged in the direction B to form an information record area.
The photo-detector 8 has photo-sensing elements linearly arranged in the direction B, and at least one entire information track of the information record plane of the optical ROM card 2 is focused onto the photo-detector. In reproducing the information, the information tracks in the desired band are sequentially focused onto the photo-detector 8 which the optical ROM card 2 is continuously or intermittently moved in the direction A by drive means (not shown) to read the information. When information in an other band is to be read, the optical ROM card 2 is moved in the direction B by a predetermined distance by drive means (not shown) and the information is read while the optical ROM card 2 is moved in the direction A, in the same manner as that described above.
Such an information reproducing apparatus can utilize the illumination light more effectively than an information recording apparatus of a type in which the optical axis of the illumination optical system is along the normal line to the information record plane 2b of the optical ROM card 2 and a light flux reflected in that direction is split by a half-mirror and focused onto the photo-detector. In reproducing the optical ROM card, the predetermined area on the information record plane having a certain area is focused onto the photo-detector by the focusing optical system to read the information. Accordingly, it is very important to enhance the utilization efficiency of the illumination light.
However, in such an information reproducing apparatus, when the information is to be reproduced from the optical ROM card, the transparent protective layer on the information recording plane optically functions as a parallel plate arranged obliquely to the optical system and hence astigmatism occurs in focusing the recording plane. As a result, a depth of focus is very shallow and in-focus control is difficult to attain and the resolution power is apt to be reduced.
In a parallel plate having a thickness of d and a refractive index of n', astigmatism .DELTA.p is expressed by EQU .DELTA.p=d(1-cos.sup.2 i/cos .sup.2 i'/n' cos i') (1) EQU sin i=n' sin i' (2)
where i is an exit angle of the light (corresponding to .theta. in FIG. 7) and i' is a refraction angle in the parallel plate.
Accordingly, where the thickness d of the transparent protective layer 2c of the optical ROM card 2 is 400 .mu.m, the refractive index n' of the protective layer is 1.6 and the angle .theta. in FIG. 7 is 30.degree., then i' is 18.2.degree. and the astigmatism .DELTA.p is approximately 44 .mu.m.
In the reproducing apparatus of the optical ROM card, a focus error in the direction of optical axis of the information recording plane 2b when the card is carried is in the order of 100 .mu.m. Since the astigmatism is relatively large compared to the focus error, the effect on the depth of focus is large, the in-focus control is difficult to attain and the reliability of information reproduction is reduced.